# setvbuf

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< c‎ | io

Defined in header ``<stdio.h>`` |  |
int setvbuf( FILE * stream, char * buffer, int mode, size_t size ); |  | (until C99)
int setvbuf( FILE *restrict stream, char *restrict buffer, int mode, size_t size ); |  | (since C99)
#define _IOFBF /*unspecified*/ #define _IOLBF /*unspecified*/ #define _IONBF /*unspecified*/ |  |

Changes the buffering mode of the given file stream `stream` as indicated by the argument `mode`. In addition,

- If `buffer` is a null pointer, resizes the internal buffer to `size`.
- If `buffer` is not a null pointer, instructs the stream to use the user-provided buffer of size `size` beginning at `buffer`. The stream must be closed (with fclose) before the lifetime of the array pointed to by `buffer` ends. The contents of the array after a successful call to `setvbuf` are indeterminate and any attempt to use it is undefined behavior.

### Parameters

stream | - | the file stream to set the buffer to
buffer | - | pointer to a buffer for the stream to use or null pointer to change size and mode only
mode | - | buffering mode to use. It can be one of the following values:
_IOFBF | full buffering
_IOLBF | line buffering
_IONBF | no buffering
size | - | size of the buffer

### Return value

​0​ on success or nonzero on failure.

### Notes

This function may only be used after `stream` has been associated with an open file, but before any other operation (other than a failed call to setbuf/`setvbuf`).

Not all `size` bytes will necessarily be used for buffering: the actual buffer size is usually rounded down to a multiple of 2, a multiple of page size, etc.

On many implementations, line buffering is only available for terminal input streams.

A common error is setting the buffer of stdin or stdout to an array whose lifetime ends before the program terminates:

    int main(void) {
        char buf[BUFSIZ];
        setbuf(stdin, buf);
    } // lifetime of buf ends, undefined behavior

The default buffer size BUFSIZ is expected to be the most efficient buffer size for file I/O on the implementation, but POSIX fstat often provides a better estimate.

### Example

One use case for changing buffer size is when a better size is known. (This example uses some POSIX function, e.g. `fileno`. See also SO: #1 and #2).

Run this code

    // Make some POSIX functions, such as `int fileno(FILE*)`, visible:
    #define _POSIX_SOURCE

    #include <stdio.h>
    #include <stdlib.h>
    #include <sys/stat.h>

    int main(void)
    {
        FILE* fp = fopen("/tmp/test.txt", "w+");
        if (fp == NULL)
        {
            perror("fopen");
            return EXIT_FAILURE;
        }

        struct stat stats;
        if (fstat(fileno(fp), &stats) == -1) // POSIX only
        {
            perror("fstat");
            return EXIT_FAILURE;
        }

        printf("BUFSIZ is %d, but optimal block size is %ld\n", BUFSIZ, stats.st_blksize);
        if (setvbuf(fp, NULL, _IOFBF, stats.st_blksize) != 0)
        {
            perror("setvbuf failed"); // POSIX version sets errno
            return EXIT_FAILURE;
        }

        int ch;
        while((ch=fgetc(fp)) != EOF); // read entire file: use truss/strace to
                                      // observe the read(2) syscalls used

        fclose(fp);
        return EXIT_SUCCESS;
    }

Possible output:

    BUFSIZ is 8192, but optimal block size is 65536

### References

- C17 standard (ISO/IEC 9899:2018):

  - 7.21.5.6 The setvbuf function (p: 225)

- C11 standard (ISO/IEC 9899:2011):

  - 7.21.5.6 The setvbuf function (p: 308)

- C99 standard (ISO/IEC 9899:1999):

  - 7.19.5.6 The setvbuf function (p: 273-274)

- C89/C90 standard (ISO/IEC 9899:1990):

  - 4.9.5.6 The setvbuf function

### See also

setbuf | sets the buffer for a file stream (function)
C++ documentation for setvbuf

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